Fix infinite recursion when unpickling a mapping type instance

[batlock666]

I noticed that unpickling mapping type instances led to infinite recursion. This happened when I tried to cache search results in Django (using django.core.cache).

Reproducing the problem:

# mt is a mapping type instance
import pickle
pickled = pickle.dumps(mt, 2)
unpickled = pickle.loads(pickled)

This should lead to a RuntimeError because of the infinite recursion. The fix prevents this from happening.

[willkg]

Can you add a test case for this?

[willkg]

Nevermind. I wrote one.